1. Field of the Invention
This invention relates to helicopter rotor blades.
In operation, vortices trailed from the tip region of a helicopter rotor blade as it rotates in the rotor disc are intersected by the following blades resulting in an impulsive blade/vortex interaction noise known as blade slap. Often the strongest interaction occurs due to a blade intersecting its own trailed vortex which may lie parallel to the span of the blade in the advancing rear quadrant of the rotor disc.
2. Description of the Prior Art
U.S. Pat. No. 4,046,336 discloses a rotor blade having a sub-wing ("tip vane") structure at its tip intended to divide the vortex into approximately equal vortices each containing only half the peak velocity of the single vortex in order to reduce pressure transients effective on the blade and to reduce the acoustic impulse that generates blade slap.
In one arrangement, the prior art blade includes a small sub-wing extended from the blade tip and having a span length between about 0.50 to 0.25 the local blade chord length and a chord length of about 0.20 the local blade chord length.
In another arrangement, the prior art blade includes a split tip portion comprising an upswept leading edge portion and a downswept trailing edge portion, the span of the respective portions and the deviation angle being selected so that the spanwise spacing between the tips is again between about 0.50 and 0.25 the local blade chord length. Similarly, the chord length of the leading edge portion is about 0.20 the local blade chord length.
In order to generate vortices of approximately equal strength we believe it to be essential to ensure that aerodynamic load is carried smoothly from the blade on to the sub-wing structure and that the sub-wing is of sufficient size to carry adequate loading. Due to the small chord and spanwise lengths of the sub-wing arrangement of the prior art rotor blade and the abrupt change in aerofoil section between the blade and the sub-wing which may prevent efficient load transfer, we consider it unlikely that the two tip vortices would be of similar strength and more likely that the device would encourage the formation of a main vortex and an associated smaller vortex issuing from the end of the sub-wing and which would rotate at a significant rate around the main vortex.
This feature results from the short span length of the sub-wing arrangement and the corresponding short distance between the tips of the leading and trailing portions of the split tip arrangement. However, the short span length and distance between the split tips is stated to be intended to ensure that the two vortices remain separated for a distance equal to about 30 chord lengths but re-combine into one vortex with a diffused core before it is encountered by the following blade.
The sub-wing of the prior art is untwisted and it is believed this could lead to flow separation on the upper surface in the area of the blade/sub-wing junction which may result in increased drag load during operation.
An objective of this invention is to provide a helicopter rotor blade which generates during operation two approximately equal strength vortices which remain discrete for as long as possible. Another objective is to provide such a rotor blade which would maintain attached flow over the surface of a vane tip to avoid a drag penalty.